1. Field of the Invention
The present invention relates in general to a distributed constant circuit board, and more particularly to such a distributed constant circuit board which contains an internal distribution of conductor patterns for a microwave integrated circuit (MIC) and an electric filter, for example.
2. Discussion of the Prior Art
Keeping pace with recent developments of various microwave devices such as mobile telephones and pocket or portable telephones, there has been a growing need for high-quality high-performance electronic components used in such devices. In particular, printed-wiring or -circuit boards for high-frequency circuits or distributed constant circuits, which are used in these electronic components, need to satisfy the following three requirements. First, the distributed constant circuit board should use electric conductors having a comparatively low resistivity, since a high resistivity of the conductors results in reduction in the Q value of a resonance circuit or inductor, and consequent increase in the transmission loss of the conductors. Secondly, the conductors should be distributed in a multi-level or multi-layered structure of the board, to minimize the size of the resultant electronic components. Thirdly, the resonance circuit, inductor and capacitor formed on the distributed constant circuit board should have sufficiently low temperature coefficients, so as to minimize changes of the physical quantities with respect to temperature.
The conventional distributed constant circuit board of the type indicated above uses ceramic substrates or resin substrates for carrying conductor patterns. The resin substrates have known drawbacks such as: difficulty in forming multi-level or multi-layered conductor circuit patterns; comparatively low dielectric constant; and comparatively high temperature coefficient of the dielectric constant. These properties associated with the dielectric constant lead to low operating reliability of the board.
An alumina substrate is known as one kind of ceramic substrate, which has a dielectric constant whose temperature coefficient is relatively high. More specifically, when a resonance circuit is formed using the alumina substrate, the resonance frequency of the circuit has a negative temperature coefficient as low as about -60 ppm/.degree. C. Further, the alumina substrate has a relatively high sintering temperature which limits the choice of the materials usable for the conductor patterns incorporated or buried therein. Namely, the alumina substrate should be used with tungsten, molybdenum and other conductors which have sufficiently high melting point and resistivity. There are also known ceramic substrates formed of forsterite or steatite, which have excellent high-frequency characteristics. However, the dielectric constants of these ceramic materials also have relatively high temperature coefficients. Therefore, the resonance frequency of a resonance circuit formed on these ceramic substrates tends to have a greatly negative temperature coefficient, as in the case where the alumina substrate is used. Further, the high sintering temperatures of forsterite and steatite limit the choice of the materials usable for the multi-level conductors, to tungsten, molybdenum or other conductive materials having a sufficiently high resistivity.
It is also known to use substrates having a relatively low sintering temperature, which are co-fired with multi-level conductors such as Ag or Cu which have a relatively low resistivity. In this case, the co-firing temperature may be lowered below the melting point of the conductors. The thus formed distributed constant circuit board also has a resonance frequency whose temperature coefficient is largely on the negative side.
For fabricating a high-frequency circuit board, it is known to form the substrates of a ceramic material which has a relatively high dielectric constant, such as those containing BaO-TiO.sub.2, MgO-CaO-TiO.sub.2 or SnO.sub.2 -ZrO.sub.2 -TiO.sub.2, for example. These ceramic materials have a comparatively low temperature coefficient, and excellent microwave dielectric characteristics. However, the ceramic substrates have high sintering temperatures and cannot be used for providing multi-level conductors principally made of low-resistance metals such as Ag and Cu which have low melting points.